The projects proposed here are designed to assess the role of proteins in the lung soluble fraction that catalyse the transfer of phosphatidylcholine. We have demonstrated the presence of a unique phosphatidylcholine exchange protein in sheep lung soluble fraction with an isoelectric point of 7.1 and a molecular weight of 21,000. It is highly specific for the transfer of phosphatidylcholine and will not catalyse the transfer of phosphatidylethanolamine or phosphatidylinositol. Experiments will be conducted to determine if this protein has a role in the lung surfactant system.